Hydrolysis-precipitation studies of aluminum(III) solutions. 3. The role of the sulfate ion

Abstract

The experimental titration curve (pH vs OH/Al) for acidified Al(III) solutions in the presence of sulfate ions differs from that measured in nitrate solutions. Precipitation occurs at a much earlier stage (OH/Al ⋍ 0.4) and the characteristic second plateau disappears except at very low SO 4 2− concentrations. Tracer experiments show most, if not all, the sulfate ions abstracted from solution during precipitation to be adsorbed. No evidence is found for the existence of basic sulfates in the precipitate. X-Ray and infrared studies combined with chemical analysis indicate the formation of a strongly hydrated hydroxide which transforms on increasing the pH to a poorly crystallized boehmite. The experimental titration curve is compared with a calculated curve based on simple thermodynamic considerations featuring a solubility product and the presence of monomeric species ( AlOH 2−). The role of sulfate is seen as that of a catalyst which removes the free energy barrier to the orientation and ordering of plate-like, highly charged, polynuclear complexes into growing solid particles. A comparison of the precipitation behavior in sulfate solutions with that in nitrate or chloride solutions allows one to measure the supersaturation built up in the latter systems and which eventually results in the nucleation and initial growth of the solid phase on the second plateau.